Presently, a water absorbing agent containing, as a main component, a water absorbent resin in addition to hydrophilic fibers such as pulp fibers is widely used as a constituent material of an absorbent article such as a disposable diaper, a sanitary napkin, and an incontinence pad.
As known absorption performances that the water absorbent resin is desired to have, there are a water absorption capacity without load, a water absorption capacity under load, a liquid permeability, a water absorbing speed, a water soluble component, a gel strength, etc. Further, many proposals have conventionally been made as to methods for measuring these physical properties and methods for improving the physical properties (methods for producing a water absorbent resin), and in addition, parameter patents (relevant to a water absorbent resin or a water absorbing agent) specifying the above physical properties.
As additional performances that are recently attracting attention other than the above physical properties, there are a gel stability (urine resistance), a powder fluidity in a moist state, a coloration resistance, a resistance to coloration over time, break resistance, a dust suppression property, etc. Many proposals have been made as to a method of maintaining essential absorption performances that a water absorbent resin is desired to have as well as having the above additional performances.
For example, Patent Literature 1 proposes an absorbent resin particle containing a urethane resin having an anion functional group, as an absorbent resin particle having a break resistance and a dust suppression property, and discloses an absorbent resin particle (i) that has a higher break resistance as compared to a conventional absorbent resin particle, (ii) that does not easily generate dust due to a fracture or a breakage, and (iii) that exhibits excellent absorption performances.
Patent Literature 2 discloses a powder absorbent polymer obtained by bonding a water-absorbing polymer particle and a fine particle having a specific particle diameter with use of a thermoplastic adhesive, as a method of providing the water-absorbing polymer particle that has excellent absorption performances and that generates less dust.
Patent Literature 3 discloses a modified water absorbent resin particle for hygiene products which water absorbent resin particle is obtained by subjecting particles made of water absorbent resin to treatment at a normal temperature with liquid organic polysiloxane. Patent Literature 3 discloses such a water absorbent resin particle, as a water absorbent resin particle that has (i) a high absorbing speed as a result of water absorbent resin particle modification that promotes uniform permeation of water, urine, menstrual blood, etc. into the water absorbent resin particle, (ii) a low moisture absorption blocking rate, (iii) a low dust generation level, and (iv) an excellent initial absorption amount under load and an excellent absorption capacity under load.
Patent Literature 4 discloses a water absorbent resin composition containing a water absorbent resin particle, a nitrogen-containing ketone compound (note that this ketone compound does not have a carboxyl group), and a quadrivalent water-soluble multivalent metal salt. Patent Literature 4 discloses such a water absorbent resin composition, as a water absorbent resin composition (i) that is excellent in absorption capacity, liquid permeability/diffusion property, and fluidity in a moist state, (ii) that has a high damage resistance, (iii) that has an excellent dust suppressing effect, and (iv) that does not easily cause segregation of an added metal compound and/or permeation of the added metal compound into the water absorbent resin particle.
Patent Literature 5 discloses a water absorbent resin composition containing a bivalent or trivalent water-soluble multivalent metal salt in place of the quadrivalent water-soluble multivalent metal salt in Patent Literature 4.
Patent Literature 6 proposes a water absorbent resin having a surface fused with a metal compound. Meanwhile, Patent Literature 7 discloses dentrimer as a dust suppressing agent.
Patent Literatures 8 through 11 disclose various dust suppressing agents for water absorbent resins. In particular, Patent Literature 8 discloses, as a dust suppressing agent, a lower aliphatic polyol whose average molecular weight is over approximately 200, a lower polyalkylene glycol (polyethylene glycol) whose average molecular weight is in a range of approximately 400 to approximately 6000, or a propylene oxide adduct (VORANOL (registered trademark, manufactured by Dow Chemical Company)) of glycerol or polyethylene glycol whose average molecular weight is in a range of approximately 400 to approximately 6000.
Patent Literatures 12 and 13 each disclose a water absorbent resin obtained by immobilizing multivalent metal salt powder with a binder.
Patent Literature 14 discloses a water-absorbing polymer having a high liquid permeability (GBP), which water-absorbing polymer is obtained by using 0.01 wt % to 5 wt % of insoluble inorganic powder and a water penetration modifier such as polyethylene glycol for surface crosslinking.
Patent Literature 15 discloses a method of reducing a dust content of a superabsorbent polymer particle composition, according to which method a contact is made with an aqueous solution of C3-C6 diol and a crosslinking compound.
Non-Patent Literature 1 discloses preparation of a water absorbent resin with use of an acrylic acid containing a predetermined amount of p-methoxyphenol (MEHQ) that is a polymerization inhibitor, and also discloses that an amount of residual MEHQ is 16 ppm to 151 ppm in a commercial water absorbent resin.
As described above, the conventional techniques have disclosed a method of reducing dusts of a water absorbent resin and a method of immobilizing fine particles, for example, a method of immobilizing a metal salt to a water absorbent resin by addition of polyethylene glycol, methoxypolyethylene glycol, polyol polyether, or the like to a surface of the water absorbent resin. However, keeping an effect of such a method for a long term has not been known as a problem.
Further, an effect, etc. caused by localization of the polymerization inhibitor on a surface of a particulate water absorbing agent has not been known. This is because the polymerization inhibitor is contained in a monomer and therefore, evenly present inside the water absorbent resin after polymerization.